The rapid development of high-precision time and frequency transfer techniques allow the real-time comparison of remote hydrogen masers (H-masers). Since the year of 2013, we have been working on building a fiber-based frequency synchronization network in Beijing, using our homemade frequency transfer devices and buried city fiber links. So far, frequency signals of 3 Hmasers from 2 different institutes are transferred to our laboratory in Tsinghua University and are compared with the local Hmaser in real time, forming up a clock ensemble of 4 clocks from 3 places. Using this frequency network, we studied the correlation of the pair of co-located H-masers. With our measurement we found that the correlation is at the magnitude of 10?30 and only shows up after the averaging time is larger than 103s.